Modern gas turbine engines which are used on aircraft are technically very complex machines. The high technology is especially apparent in the portions of the engine which operate at elevated temperature. Not only are such components subjected to degrading temperatures, corrosion, and thermal fatigue, but they must be made as light as possible in design, according to the dictates of their use in aircraft. While most of the components in modern gas turbine engines have long lives, there is inevitably some degradation with prolonged use. Since the components are rather costly because of the high technology them embody, there is a great desire to repair or rework parts to the extent this is possible.
One of the components which falls within the foregoing context is the combustor liner of the engine. The combustor or burner is the portion of the engine where fuel is mixed with compressed air and combustion takes place. The combustor liner, illustrated in the Figures, is an annular structure having an inner and an outer wall; it is closed at one end where there are fuel injection nozzles, and open at the other end where the hot products of combustion are discharged toward the turbine section of the engine. On a large gas turbine engine this liner might have an outside diameter of 39 inches, an inside diameter of about 23 inches and a length of about 17 inches. Typically, it will be made of a wrought nickel superalloy, such as Hastelloy X, having a typical thickness of about 0.045 inches. It will thus be appreciated that the structure is relatively light in construction for its size. During use, thermal fatigue cracks sometimes occur and, more generally, there is warping and distortion of the structure.
However, in repair operations heretofore it has been a problem that the degree of distortion resulting from use or welding was substantial. This has required corrective deformations of portions of the liner by as much as a half inch in space. And, despite fixturing of the portions of the apparatus, it is commonly observed that the combustor liner after welding does not meet the required dimensions, as they were dictated by the fixture. An obvious approach which has been taken has been to attempt to cold form the structure to bring it into compliance with the required dimensions. But, given the complexity of the liner structure, this is not so easily achieved. Either the requisite dimensions cannot be achieved owing to consequential secondary distortion of a remote portion of the structure, or there is cracking of portions of the structure. While Hastelloy X is commonly thought to be a material which can be cold formed, it appears that the complexity of the structure and perhaps changes in the character of the material due to its prolonged use make cracking more likely than would appear apparent at first. This observation might lead one to consider hot working of the structure. But the physical size of the structure and the resultant necessity that the tooling being used in straightening be heat resistant makes it somewhat infeasible.